Frost and/or freeze can cause extensive damage to agricultural products. A significant factor in frost formation is radiative cooling from the earth's surface and crops growing thereon. Such radiative cooling results in heat loss to the atmosphere, and is particularly prevalent on windless clear nights. Hence, several methods have been used to protect plants from damage caused by frost and/or freeze.
For example, an early preventative measure involved the use of smudge pots in orchards. The smudge pot operated by burning oil in a container. Heat was thus produced to raise the temperature in the orchard to above dangerous levels. The smoke generated by the burning also provided a blanket of insulation that helped to retain the heat in the orchard.
Another conventional technique is to spray water over the orchard or crops. This increases the dew point of the air which helps the air to trap and hold the earth's radiant heat. Also, condensation of the water vapor on the fruit releases energy (i.e., the heat of fusion), thus raising the temperature of the fruit.
Other methods of forming insulative fog and/or mist layers are also known. For example, U.S. Pat. No. 5,052,618 to Carlon et al. discloses a method of protecting plants from frost by using a jet engine to generate an aerosol of a first liquid microencapsulated in a second liquid. The aerosol is dispersed in a mist about the plants to be protected from frost. The mist acts as a protective radiation barrier for the plants. U.S. Pat. No. 4,352,458 to Masel discloses a device for generating and transporting over substantial distances a jet of hot combustion gases having entrained therein condensed water. The generated jet of gases and entrained condensed water provides a vapor mist, and the device is movable from one location to another in a manner so as to maintain a cloud of vapor in a desired orientation with respect to growing crops and over a desired amount of time.
Yet another technique involves using wind machines to generate a wind stream which forces cold air up and warm air down into the orchard. Such wind machines can also be used in conjunction with means for introducing a moisture mist into the generated wind stream to further protect crops from frosting during cold nights.
Other methods include, for example, use of a pulse-jet engine as described in U.S. Pat. No. 5,285,769 to Wojcicki that generates individual bursts of pulses of heat over a pulse cyclic period. The combustion products are in the form of a hot jet which is exhausted to the orchard to raise the temperature and prevent or reduce the risk of freeze damage to fruit trees or similar crops.
Yet another technique for protecting crops against frost damage is direct water sprinkling of the crops. In this method, the radiative heat loses are partially compensated for by the latent heat produced by freezing of the water drops. The Masel patent, discussed above, notes that this technique suffers from the disadvantage that significant amounts of ice, having a conductivity four times that of water, are formed on the crops, for example, on the leaves of fruit trees. Therefore, the ice thus formed enhances the cooling of the leaves. Furthermore, the weight of the ice can damage the plant.
Water has also been combined with other components to form freeze preventative and/or protective compositions for direct application to plant surfaces. Such compositions for direct application to plants have mainly involved the use of freezing point depressants, such as monohydric alcohols, small chain dihydroxy and polyhydroxy alcohols, such as propylene glycol, polyalkyl glycols, and so forth, and other agents such as cross-linked polyacrylic acid.
For example, U.S. Pat. No. 5,653,054 to Savignano et al. discloses a composition for preventing or retarding frost formation on grass or leafy plants comprising a mixture of water, a water-soluble freezing point depressant such as propylene glycol, and a water dispersible thickening agent such as a cross-linked polyacrylic acid polymer. The composition is applied by spraying and, preferably, just prior to expected frost onset. The composition protects plants by lowering of the freezing point of moisture that condenses or collects overnight on plant surfaces treated with the composition.
U.S. Pat. No. 5,668,082 to Miller et al. discloses a "sticker" composition and/or "sticker/spreader" adjuvant combined with active materials used in the agricultural and horticultural industry. The composition comprises a low molecular weight polyolefin hydrocarbon resin and an active material such as insecticides, herbicides, fungicides, rodenticides, nutrients, plant growth regulators, pheromones and defoliants. The patent broadly discloses that the invention can be extended with beneficial results to the retention and dispersion of various protective coatings on numerous fruit and vegetable varieties for protection from excessive heat, cold or sun exposure. However, the patent discloses no compositions specifically designed for protection of plants from frost and/or freeze.
U.S. Pat. No. 5,185,024 to Siemer et al. discloses application of cross-linked hydrated polyacrylamide and poly(ammonium) acrylate hydrogels to agricultural soils or crops by spraying. The hydrogels are described as typically rigid and insoluble and may include such additives as surfactants, plant micronutrients or macronutrients, pesticides, plant growth regulators, freezing point depressants, microbes and colorants. The patent discloses that hydrogels can be used as soil amendments and for the treatment of crop foliage. For example, use of hydrogels can substantially reduce the use of irrigation water by retention of moisture in the soil or around crop roots. When surfaces of living or cut plants are coated with hydrogels, transpiration of water through the leaves is limited, thus prolonging survival of living leaves in plants stressed for water, and prolonging the period of pleasing appearance of cut plants or flowers. The patent also discloses that hydrogels have been applied to the surface of crop plants to protect the foliage from frost damage by taking advantage of the high latent heat capacity of water to protect the plants. The patent discloses specific examples of spraying the hydrogels onto turf subjected to hot and dry weather conditions and of spraying the hydrogels onto pinto bean seeds or on the surface of the sand after planting the beans.